The present invention relates to a reactor cooling system for nuclear power plants, and more particularly to a reactor cooling system for boiling water reactors which is suitable to remove residual heat in a reactor core and cool the reactor core.
One of known reactor cooling systems for boiling water reactors is described in Application for License of Modifying Reactor Installation of Nuclear Power Plant at Kashiwazaki Kariba, Japan, and shown in FIG. 5.2-1. This system is of an emergency core cooling system (ECCS) for an advanced boiling water reactor (ABWR). The construction of FIG. 5.2-1 is shown in FIG. 3 in the simplified form.
Referring to FIG. 3, the ECCS comprises three independent sections, i.e., sections I, II and III. While each section employs an in-plant power source as a normal power source, emergency diesel generators (D/G) 31a to 31c are also provided in the respective sections against cut-off of the in-plant power source. The sections include high pressure core injection systems (HPCF's) 33, 32b, 32c and low pressure core injection systems (LPFL's) 34a to 34c, respectively, these core injection systems being provided in one pair per section and independently of one another. The high pressure core injection system 33 in the section I also serves as a reactor core isolation cooling system (RCIC). Each pair of systems in the sections has capacity not less than 50% of the capacity necessary to maintain submergence of a core in a pressure vessel 2. Accordingly, in case of a single failure in which any one of the sections loses its entire function, the remaining two sections would have total capacity not less than 100% of the capacity necessary to maintain the submergence of the core, thereby assuring of safety.
The HPCF's 32b, 32c and the RCIC 33 are each actuated upon a water level set to a position lower than a normal water level for injecting water into the pressure vessel 2 and suppressing a reduction of the water level even in a condition that the reactor core pressure is still high. The LPFL's 34 are each actuated upon a water level lower than the actuation level of the HPCF's 32b, 32c and the RCIC 33 for injecting water into the pressure vessel 2 when the reactor core pressure becomes low, thereby maintaining the submergence of the core for a long term after the accident.
Supposing now a breakage of any HPCF 32 which is most serious accident in the reactor core cooling, for example, a breakage of the HPCF 32b in the section II, the HPCF 32b stops its function. Additionally, supposing the occurrence of a single failure, for example, a failure of the emergency diesel generator (D/G) 31c in the section III, this would be a very serious accident that the ECCS in the section III stops its entire function at the same time. Even in such an event, the remaining total capacity not less than 100% of the capacity necessary to maintain the submergence of the core would be ensured by the RCIC 33 and the LPFL 34a of the section I and the LPFL 34b of the section II, thus giving the ECCS with a capability of keeping the core submerged in water.
However, the above-mentioned prior art has suffered from the following problems.
In the above-mentioned prior art, all the cooling systems in the three sections of the ECCS are constituted by pumps as dynamic equipment. In an attempt to check the cooling systems in any one section for maintenance during normal operation, if a single failure should occur in another section, only one section would remain operable, making the capacity of the ECCS down to just 50% or more of the capacity necessary to maintain the submergence of the core. Accordingly, the conventional reactor cooling system cannot be checked for maintenance during normal operation. For the same reason, the conventional reactor cooling system cannot deal with incapability of functions in two or more sections, i.e., the occurrence of multiple failures.
As another known related technique, JP, A, 54-36490 discloses a core cooling system for boiling water reactors in which a plurality of dynamic core injection systems and a plurality of static core injection systems are combined with each other. The static core injection systems in this known prior art employ, as a water source, water in an equipment storage pit positioned higher than a pressure vessel containing a core, hence a sufficent amount of water is present. However, it is not clarified whether the total capacity of the static core injection systems is set to be not less than 100% of the capacity enough to achieve submergence of the core. There is thus no full conviction that in case of multiple failures in which plural dynamic core injection systems lose a capability of injecting water, the core would be surely submerged in water by the static core injection systems.
As still another known related technique, JP, A, 63-30786 discloses a core cooling system for pressurized water reactors in which a dynamic core injection system and a static core injection system are combined with each other. The static core injection system in this known prior art employs, as a water source, an injection tank in which water is enclosed under a high pressure.